This invention relates to an injector drive circuit for driving an injector for feeding an internal combustion engine with fuel, and more particularly to an injector drive circuit which is suitable for use for an internal combustion engine and constructed so as to directly inject fuel into a cylinder of the internal combustion engine.
An injector or electromagnetic fuel injection valve is typically used as a means for feeding fuel to an internal combustion engine.
Such an injector generally includes a valve for operating an injection port and a solenoid coil for driving the valve and is constructed so as to open the valve to inject fuel into a fuel injection space such as an interior of an air inlet pipe, an inner space of a combustion chamber or the like when a drive current of a predetermined magnitude is fed to the solenoid coil.
The injector is mounted on the internal combustion engine while keeping the injection port communicating with the fuel injection space of the internal combustion engine to which fuel is to be injected such as the interior of the air inlet pipe, an interior of a cylinder or the like and is fed therein with fuel under a predetermined pressure from a fuel pump. A pressure of fuel fed to the injector is kept constant by means of a pressure regulator. The solenoid coil is connected to an injector drive circuit. The injector drive circuit functions to feed a drive current to the solenoid coil when it is fed with an injection command signal from a control unit for controlling the injector.
In order to appropriately control the injector, it is required to displace the valve from a closed position to an open position set, to thereby reduce valve opening transition time during which the valve is kept open, as soon as the drive circuit is fed with the injection command signal. Also, it is required to keep the valve at the open position during a period of time for which the drive circuit is fed with the injection command signal, after valve opening operation is completed.
In order to reduce valve opening transition time of the injector supposing that a drive voltage is rendered constant, it is required to increase the number of turns of a coil while increasing a diameter of a wire for the coil to reduce a resistance of the coil per unit length thereof, to thereby permit flowing of a drive current increased in magnitude. However, such construction of the solenoid coil causes the solenoid coil to be increased in volume, leading to large-sizing of the injector, so that a space increased to a degree sufficient to permit the injector to be mounted on the internal combustion engine must be ensured. In particular, when fuel is to be directly injected into the cylinder, it is highly difficult to ensure the space which permits the injector to be mounted on the engine, thus, it is inevitable to prevent large-sizing of the injector.
In order to avoid large-sizing of the injector, it is required to reduce a diameter of a wire conductor for the solenoid coil and decrease the number of turns of the coil to the utmost. However, in order to rapidly carry out operation of opening the injector by means of the solenoid coil thus constructed, it is required to construct the injector drive circuit in a manner to increase a drive voltage applied from a power supply section of the injector drive circuit to the solenoid coil, to thereby ensure rapid rising of a drive current fed to the solenoid coil during starting of the injector and fully increase the drive current. A level of the drive voltage applied to the solenoid coil during the starting depends on a magnitude of initial load of a return spring used for the injector or a magnitude of elastic force of the return spring provided when the solenoid coil is kept from being excited.
When fuel is injected into the air inlet pipe of the internal combustion engine, an environmental pressure of the injector is at a level of an atmospheric pressure or below. Also, it is as low as about 0.5 MPa even when the engine is mounted thereon with a supercharger. Thus, in the injector used for injection of fuel into the air inlet pipe, it is not required to increase initial load of the return spring to a significantly increased level, so that the solenoid coil may be driven at a power supply voltage of 12 V applied from a battery thereto while being small-sized.
On the contrary, the injector for direct injection of fuel into the cylinder is increased in environmental pressure, therefore, it is required to increase initial load of the return spring to a considerable level.
Thus, in order to reduce valve opening transition time of the injector to improve controllability of a fuel injection rate when the injector operated under an increased environmental pressure is driven, it is required that a DC-DC converter for increasing a voltage outputted from a battery is arranged so as to provide the injector with a drive voltage therefrom, as disclosed in Japanese Patent Application Laid-Open Publications Nos. 144622/1997 and 26701/1995. Unfortunately, this causes a structure of the power supply section of the injector drive circuit to be complicated, leading to an increase in manufacturing cost of the injector drive circuit.
Another injector drive circuit is disclosed in Japanese Patent Application Laid-Open Publication No. 61125/1996. The injector drive circuit disclosed is constructed so as to charge a capacitor by means of a fly-back voltage induced across a primary winding of an ignition coil of an ignition device when a current flowing through the primary winding of the ignition coil is interrupted at an ignition position of an internal combustion engine, to thereby apply a voltage at an increased level across the capacitor to a solenoid coil, resulting in driving the injector at an increased speed. However, such construction of charging the capacitor by means of a high voltage induced across the primary winding of the ignition coil at an ignition position of the engine tends to deteriorate ignition performance of the ignition coil, resulting in being disadvantageous, because the capacitor acts as a load for a primary circuit of the ignition coil.